drivers/savagefb: use mdelay instead of udelay

Long delays need to use mdelay on architectures such as ARM
that cannot compute long timeouts in udelay.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>